Underpayment Calculator

Your contracted allowed amount (USD) What your fee schedule says this code should be allowed at, per unit.

Allowed amount on the remittance (USD) The allowed amount the payer actually applied — not the paid amount, and not your billed charge.

Units on the claim Leave blank for 1.

Claims per month with this code Optional — used to show what the variance costs at your volume.

Variance per claim

Annual exposure

Enter your contracted rate and the allowed amount from the remittance.

Why Underpayments Are the Hardest Money to Find

A denial announces itself. It arrives with a reason code, lands in a work queue, and somebody looks at it. An underpayment does none of that. The claim pays. It appears on the remittance alongside every other paid claim, it reconciles, and nothing anywhere flags that the payer allowed 86 dollars on a code your contract prices at 94.

That is the entire problem. Underpayment detection is not a follow-up activity — there is nothing to follow up on. It only exists if somebody deliberately compares allowed amounts against the fee schedule, code by code. Most practices never do, which is why underpayments typically run for years before anyone notices, and why they are the single largest pool of quietly recoverable revenue in most practices.

The other reason they persist: a contracted rate does not drift back on its own. If the payer has the wrong fee schedule loaded, every claim for that code is underpaid every time until someone raises it. The loss is not a one-off; it is an annuity running in the payer’s favour.

CO-45 Is Not an Underpayment

This confusion costs practices real time, so it is worth being precise.

CO-45 is the contractual adjustment — the difference between what you billed and what your contract allows. If you bill 300 dollars for a code contracted at 94, CO-45 writes off 206. That is normal, expected, and appears on essentially every in-network claim. It is not a problem and it is not appealable.

An underpayment is different: the allowed amount itself is below your contracted rate. CO-45 will still appear on that claim and will look completely ordinary — it just writes off a larger amount. Nothing distinguishes the two on the remittance, which is precisely why you have to compare against the fee schedule rather than reading the adjustment codes.

The Five Causes Worth Checking First

An outdated fee schedule. You renegotiated, the contract took effect, and the payer never loaded the new rates. This is the most common cause and the easiest to prove — you have the signed contract.

Modifier pricing. Reduced-value modifiers applied where they should not be, or bilateral and assistant modifiers priced at the wrong percentage.

Multiple procedure reduction. MPPR applied to a second procedure that was not subject to it, or applied at the wrong percentage.

Place of service pricing. The claim priced at the facility rate when it should have been non-facility, which on office-based procedures is a substantial difference. See the RVU calculator for what the two rates should be.

Sequestration mistaken for underpayment. On Medicare, a 2 percent reduction is statutory, not an error. Check it with the sequestration calculator before disputing it.

How to Work an Underpayment

Start with the pattern, not the claim. One underpaid claim is an anecdote; the same code underpaid across 40 claims is a fee schedule problem, and that is what gets fixed. Pull your highest-volume codes, compare the allowed amounts on recent remittances against your contract, and look for codes where the variance is consistent. Consistency is the signal — random variance is usually modifier or MPPR handling, while a fixed gap on every claim is a loaded rate.

Then dispute it as a payment dispute rather than an appeal. You are not arguing about medical necessity or coding; you are asserting that the payer did not honour a contracted rate, and the evidence is the contract itself. Reference the specific rate, the specific claims, and the total. Note that payment dispute windows are often shorter than appeal windows and usually run from the remittance date, so check the contract before assuming you have a year.

More Free Billing Tools & Guides

Underpayment work usually pairs with the RVU calculator for what Medicare should allow, the sequestration calculator to rule out the 2 percent statutory cut, CO-45 for the contractual adjustment itself, and how to read an ERA for locating the allowed amount on the remittance. For collection benchmarks see the net collection rate calculator.

Frequently Asked Questions

How do I know if a payer underpaid me?

Compare the allowed amount on the remittance against your contracted rate for that CPT code, modifier and place of service. If the allowed amount is lower than your contract says, you were underpaid. Nothing on the remittance flags this - the claim shows as paid, which is why underpayments go undetected for years.

Is CO-45 an underpayment?

No. CO-45 is the contractual adjustment - the difference between what you billed and your contracted allowed amount. That is normal and expected on every in-network claim. An underpayment is when the allowed amount itself is below your contracted rate, which CO-45 does not reveal.

Why do underpayments happen?

Most often the payer has an outdated fee schedule loaded, a renegotiated rate was never applied, modifier pricing was mishandled (reduced-value modifiers applied incorrectly), multiple procedure reduction was applied when it should not have been, or the claim was priced at the wrong place of service rate.

How far back can I recover an underpayment?

It depends on your contract and state prompt-pay law. Most payer contracts contain a reconsideration window measured in months from the remittance date, and some states set a statutory lookback. Check the contract first - the window for a payment dispute is usually shorter than the appeal window for a denial.

Is a small underpayment worth chasing?

Per claim, rarely. At volume, always. A four dollar variance on a code you bill 200 times a month is 9,600 dollars a year, and because the contracted rate does not change on its own it recurs until someone catches it. Chase the pattern, not the individual claim.

What is the difference between an underpayment and a denial?

A denial pays nothing and tells you why. An underpayment pays, just less than it should, and tells you nothing. That is why denials get worked and underpayments do not - one lands in a work queue and the other looks like a successful claim.